Automatic rain sensors for controlling the windshield wipers on vehicles have employed many moisture sensing methods including capacitance, resistance, microwave and optical means. Capacitive and resistive sensors are economical and relatively simple but require the application of special coatings to the windshield glass. Functional coatings must be abrasion resistant, aesthetically acceptable, and reliable over the long term. Methods for applying these sensor coatings generally involve techniques that interfere with the normal production process for windshield glass, such as a thin or thick film of conductive material fired into the glass surface.
Optical sensors employ infrared emitters and detectors to determine whether moisture is present on the windshield surface. The advantage of these optical sensors is that they can theoretically be adapted to any windshield without the need for special glass processing. The problem with optical sensors is functioning in direct sunlight or even clear sky conditions.
One type of optical sensor emits radiation into the window wherein it is internally reflected several times and then detected. The radiation is converted to an electrical signal for activating the windshield wipers. Such a system is described in U.S. Pat. No. 4,481,450 issued Nov. 6, 1984 in the name of Takashi Watanabe and assigned to Nippondenso Co., Ltd. This patent discloses an optical sensor on the inside of a window which emits infrared radiation directly into a material comparable with that of the window. The infrared radiation passes into the glass and is reflected several times before being sensed on the inside of the window. An electrical signal is then produced which is representative of the amount of radiation reflected and therefore the amount of moisture on the windshield. A problem with this type of sensor is that the magnitude of the radiated beam is decreased substantially after each reflection making the detection thereof difficult; manufacture of and application of the sensor to the windshield is difficult in order to ensure no air gaps between the sensor, material and glass, and if there are air gaps, refraction of the energy has not been considered. Ambient and direct light may easily saturate the detector and interfere with the operation this sensor making it inoperable.
Another type of optical sensor is described in U.S. Pat. No. 4,355,271 issued Oct. 19, 1982 in the name of Raymond J. Noack. This patent discloses two optical sensors operating from a common emitter. The sensors are spaced to take into account only reflection of the outside surface of the window. One sensor is to sense moisture, the other sensor is to sense dirt or other particles. A problem with this type of sensor assembly is that the refraction at the air-glass surface has not been accounted for. Additionally, incident sunlight falling on the photodetector would easily saturate the sensor rendering the assembly useless.